When a video signal is recorded on and then reproduced from a recording medium such as a magnetic tape, the reproduced video signal may include time base errors or fluctuations reflecting minute changes in the speed at which the video signal is reproduced. For example, through age or wear on the magnetic tape, portions of the magnetic tape may be slightly stretched or contracted, with the result that a video signal reproduced from the stretched portion takes slightly longer per horizontal line for reproduction, while a video signal reproduced from a contracted portion takes slightly less time. Other sources of time base errors include the improper functioning of the reproducing apparatus or the inherent effects of normal operation. However, the proper display of the reproduced video signal depends crucially on the vertical and horizontal synchronizing signals occurring with precisely the correct intervals therebetween, since the visible result of irregular timing is to displace what would otherwise be displayed as a purely vertical line into a line with oscillations or rapid horizontal displacements and slower decays and so time base fluctuations such as those described above, if left uncorrected, present a serious problem.
Therefore, to remove such time base fluctuations, it is known to include a time base corrector in the reproducing apparatus which reads the reproduced video signal into a memory at a variable rate reflecting the time base fluctuations included in the reproduced video signal and to read out the stored video signal at a constant rate without the time base fluctuations. Such time base correctors generally include a phase locked loop control circuit responsive to the synchronizing signals in the reproduced video signal for producing a high frequency write clock signal which includes the time base fluctuations of the reproduced video signal. The phase locked loop control circuit conventionally includes a voltage controlled oscillator producing the output high frequency signal, a frequency divider for dividing the output signal, a phase comparator which compares the phases of the frequency divided output signal and the horizontal synchronizing signals from the reproduced video signal and a low pass filter for filtering the output of the phase comparator to provide a control voltage for the oscillator. Since the output of the oscillator is controlled to reflect the varying frequency of the successive horizontal synchronizing signals, it can be the basis for the write clock signal to control time base correction. Thus, the reproduced video signal is read into the memory at times determined by the write clock signal. A corresponding read clock signal is generated from an oscillator having a known frequency and the stored video signal is read out in response thereto.
However, the use of the low pass filter to provide the control voltage inherently filters out any high frequency components of the time base fluctuations, and so this known time base corrector does not correct for such high frequency fluctuations. The reproduced video signal frequently includes both random and periodic high frequency time base fluctuations and it is known in the prior art to provide a velocity error corrector which modifies the read clock signal so as to remove both types of high frequency fluctuations. However, such velocity error correctors are generally complex, difficult to install and expensive and are disadvantageous for use in a mass produced item such as a video tape recorder. Consequently, many video tape recorders do not contain a velocity error corrector and so do not compensate for high frequency fluctuations.